There are many known welding-type systems used to provide a welding-type output or welding-type power for many known applications. Welding-type system, as used herein, includes any device capable of supplying power suitable for welding, plasma cutting, induction heating, CAC-A and/or hot wire welding/preheating (including laser welding and laser cladding), including inverters, converters, choppers, resonant power supplies, quasi-resonant power supplies, etc., as well as control circuitry and other ancillary circuitry associated therewith. Welding-type power or output, as used herein, refers to power suitable for welding, plasma cutting, induction heating, CAC-A and/or hot wire welding/preheating (including laser welding and laser cladding). Welding-type power circuit, as used herein, refers to power circuitry that receives an input power and provides welding-type power.
Examples of prior art welding-type systems include those described in Method of Designing and Manufacturing Welding-Type Power Supplies, Albrecht, filed Sep. 19, 2001, application Ser. No. 09/956,401, which issued on Mar. 30, 2004 as U.S. Pat. No. 6,713,721; Pendant Control for a Welding-Type System, L. Thomas Hayes, filed Sep. 19, 2001, application Ser. No. 09/956,502, which issued on Oct. 28, 2003 as U.S. Pat. No. 6,639,182; Welding-Type Power Supply With A State-Based Controller, Holverson et al., filed Sep. 19, 2001, application Ser. No. 09/956,548, which issued on Jun. 8, 2004 as U.S. Pat. No. 6,747,247; Welding-Type System With Network And Multiple Level Messaging Between Components, Davidson et al., filed Sep. 19, 2001, application Ser. No. 09/957,707, which issued on Dec. 30, 2003 as U.S. Pat. No. 6,670,579; Welding-Type Power Supply With Boot Loader, L. Thomas Hayes, filed Sep. 19, 2001, application Ser. No. 09/956,405, which issued on Jan. 7, 2003 as U.S. Pat. No. 6,504,131; Welding-Type System With Robot Calibration, Rappl et al., filed Sep. 19, 2001, application Ser. No. 09/956,501, which issued on Nov. 4, 2003 as U.S. Pat. Nos. 6,642,482; 6,087,626, Hutchison, et al., Jul. 11, 2000, entitled Method and Apparatus for Welding and United States Patent Publication 20120061362, entitled Method and Apparatus For Welding With Short Clearing Prediction. Each of these patents is hereby incorporated by reference.
Some such systems are used for short circuit (MIG) or short arc welding, a process where the arc alternates between a short state and arc state. A wire electrode is fed into the weldment by a wire feeder in the short circuit transfer welding process. It is consumed into the weldment via the series of alternating short circuit and arc events. This process is generally referred to as short arc welding, short circuit welding, or short circuit transfer welding. Generally, a welding machine used for short arc welding includes at least a power source, a controller and a wire feeder. A prior art short arc waveform from the Miller® RMD® process is shown in FIG. 1. The upper images show the wire, puddle and arc or short, and the lower graph shows the current command for the phases of one short arc cycle. The waveform begins with a wet phase 101 while the ball is wetting into the puddle. During a pinch phase 103 the ball begins to transfer to the puddle. During a clear phase 105 the transfer continues until the short is cleared. During a blink phase 107 the arc is reestablished. During a ball phase 109 the end of the wire melts and forms a ball, and during background phase 111 the ball continues melting and advances towards the puddle. Phases 101-105 are during the short, and phases 107-111 are during the arc.
The short circuit transfer welding process is cyclical. One cycle of the process, as described herein, begins with the beginning of a short circuit condition followed by a steady state arc condition, and is completed with the beginning of another short circuit condition. A typical cycle length is 10 msec. The electrode, and a portion of the base metal, are melted during the short circuit transfer welding process by current flowing through the electrode to the weldment. Generally, a portion of the wire material melts during the arc condition, and is transferred during the short condition.
The event of clearing the short, i.e., the transition from a short circuit to an arc, may be a most violent portion of the process and can produce spatter. The explosive nature of this event has been reduced, by lowering the magnitude of the current prior to or at the short clearing, thereby limiting the power density. Some prior art short arc systems sensed the clearing, and then lowered the current magnitude.
Better short arc systems, such as Miller® RMD®, predict the short clearing, so the current can be lowered prior to the clearing. The RMD® prediction includes monitoring the second derivative of power. The RMD® process is an improvement over standard short circuit MIG because it removes energy during the critical time of short circuit clearing. Reducing energy during the short clear reduces spatter generation, and minimizes the disturbance to the weld puddle. But, the energy cannot be reduced too low or there will not be sufficient energy to maintain the arc and grow the next molten ball of metal to be shorted in the puddle and deposited.
Even when the short is predicted by the RMD® process, it can be difficult to lower the current to a level that does not disturb the puddle or cause spatter because the current does not always reach the targeted or commanded short clear current in the brief time between the prediction of the short clearing, and when the short clearing occurs. Welder output inductors and cable inductance limit the rate of change of current. The inductance acts as an energy storage system to prevent sudden changes in current. The prediction of the short clear is often about 200-400 microseconds in advance of the clear. System inductances can delay the current from reaching its target for times significantly longer than the 200-400 microseconds from prediction to short clearing. Higher current levels means more energy gets stored in the inductance, and the delay in the response to a current command reduction is longer. The additional energy (from the actual current being higher than the commanded current) causes energy to be dissipated as the wire breaks in to an arc, resulting in more spatter and puddle disturbance than desired.
Accordingly, a welding type system that performs welding and includes a way to reduce the current before or when the short clears is desirable.